feat: support uv.lock as version-file (#379)

Adds `uv.lock` as a supported `version-file` format, alongside the
existing
`pyproject.toml` and `requirements.txt`. A `uv.lock` pins the exact
resolved
ruff version, so reading it gives a reproducible install that matches
the
project's lockfile — the single source of truth — instead of the
lower-bound /
range typically declared in `pyproject.toml`.

The parser reads the `[[package]]` entry whose `name` is `ruff` and
returns its
`version`. If ruff is absent or the file cannot be parsed, the action
warns and
falls back to `latest` (same behaviour as the other formats).

Validation:
- `npm run all` — tsc typecheck clean, biome clean, dist rebuilt, jest
65/65 pass
- Added unit tests (`__tests__/version/file-parser.test.ts`) + a
`uv.lock` fixture
- Added an integration job (`test-default-version-from-uv-lock`) that
runs the
  action against the fixture and asserts ruff 0.9.5 is installed

closes #375

__tests__/version/file-parser.test.ts

@@ -9,9 +9,11 @@ jest.unstable_mockModule("@actions/core", () => ({
   warning,
 }));
 
-const { findRuffVersionInSpec, getRuffVersionFromFile } = await import(
-  "../../src/version/file-parser"
-);
+const {
+  findRuffVersionInSpec,
+  getRuffVersionFromFile,
+  getRuffVersionFromUvLockContent,
+} = await import("../../src/version/file-parser");
 
 describe("file-parser", () => {
   beforeEach(() => {
@@ -124,5 +126,26 @@ describe("file-parser", () => {
       );
       expect(result).toBe("~0.8.2");
     });
+
+    it("reads the version from uv.lock", () => {
+      const result = getRuffVersionFromFile("__tests__/fixtures/uv.lock");
+      expect(result).toBe("0.9.5");
+    });
+  });
+
+  describe("getRuffVersionFromUvLockContent", () => {
+    it("extracts the ruff version from uv.lock content", () => {
+      const result = getRuffVersionFromUvLockContent(
+        '[[package]]\nname = "other"\nversion = "1.0.0"\n\n[[package]]\nname = "ruff"\nversion = "0.9.5"\n',
+      );
+      expect(result).toBe("0.9.5");
+    });
+
+    it("returns undefined when ruff is not in uv.lock", () => {
+      const result = getRuffVersionFromUvLockContent(
+        '[[package]]\nname = "other"\nversion = "1.0.0"\n',
+      );
+      expect(result).toBeUndefined();
+    });
   });
 });